A blade for a rotor of a wind turbine is typically produced by means of a casting procedure. The casting procedure for a new type of wind turbine blade comprises (a) producing a positive mold representing a design model of the blade which is supposed to be produced, (b) producing a negative mold representing negative copy of the positive mold and (c) producing the blade by casting an appropriate blade material into the negative mold. Thereby, each step has to be finished before starting the next step.
In this document a positive mold is also denominated as a male mold, a positive form and/or a plug. A negative mold is also denominated as a female mold, a negative form and/or a casting mold.
The positive mold respectively the plug is typically produced by means of a milling process, which is accomplished by means of a large Computer Numerical Controlled (CNC) milling machine. Thereby, due to material properties a sub-frame for the positive mold is required having the general geometrical shape of the positive mold. On the sub-frame softer composite material layers are laid down between harder curable materials like fiberglass. The softer composite material layers can be processed by a milling procedure. Because of the large dimension of the positive mold such a milling process lasts typically 8 to 10 weeks.
US 2009/0084932 A1 discloses a negative mold for casting a wind turbine blade. The negative mold comprises a plurality of spaced-apart joists, wherein each joist has an edge configuration generally corresponding to a form of the blade. The negative mold further comprises a flexible frame, supported by the edges of the joists, for shaping an exterior surface of the blade. On the flexible frame there are formed different coatings layers. (A) A first coating layer includes a rigid, semi-rigid, and/or flexible spray foam, such as a polyurethane foam. Such low density, expanding materials for the first coating (a) will fill any openings in the flexible frame, (b) provide improved structural rigidity with little increase in weight and (c) are relatively easy to machine. Once the first coating layer is applied and cured, the surface of the first layer may be cut, ground, sanded and/or otherwise formed to a shape that more-closely corresponds to the intended external shape of the blade. In particular, the first layer may be machined with computer-controlled equipment so as to provide exactly the desired shape. (B) A second coating layer may be applied as a protective layer of higher density material, such as filled or unfilled plastic resins. In addition to enhancing durability of the negative mold, the second coating layer also provides a smooth surface against which to form the blade.
When using the negative mold disclosed in US 2009/0084932 A1 for producing a wind turbine blade it is not necessary to produce a positive mold first. The disclosed negative mold can be produced directly based on three dimensional data representing the geometry of the wind turbine blade which is supposed to be casted. However, the disclosed negative mold has the disadvantage that building up the negative mold is a rather elaborate procedure.
There may be a need for a negative mold for casting a component such as a wind turbine blade, wherein the negative mold can be build up in an easy and effective manner.